The present invention relates to an apparatus and method for simultaneous estimation of Nxc3x97M signal transmission paths of an N-input M-output linear FIR system (where N=2,3, . . . and M=1,2, . . . ) such as a structure provided with pluralities of sensors and actuators and a multi-loudspeaker multi-microphone system. The invention also pertains to a recording medium with the method recorded thereon.
With recent developments in the technology of digital processing and speedups of arithmetic processing, acoustic signal processing such as sound pressure control and active noise control, originally intended for use in a single-input single-output system, is now going into use in a multi-input multi-output system. With such signal processing, the multi-input multi-output system is supplied with signals that have passed through a control filter. Since the control filter has its coefficients computed from the characteristics of the multi-input multi-output system, an exact extraction or identification of the system characteristic is needed.
A possible example of an application of such acoustic signal processing is a home theater, which is an extension of a conventional two-channel stereophonic reproduction system to a multichannel system using four or six loudspeakers. In the implementation of a sound system closer to that of a movie theater, it is necessary to identify transfer characteristics of multiple transmission paths in the listening room so as to adjust the control filter for acoustic signal processing use accordingly.
In an N-input M-output linear system it is conventional to derive transfer characteristics of Nxc3x97M signal transmission paths by dividing N one-input M-output subsystems and estimating the transfer function of each subsystem through calculation of the correlation between the input signal and each of M output signals. With this method, the transfer functions of the N subsystems are determined not simultaneously, but one after another. An example of this method is disclosed in Japanese Patent Application Laid-Open Gazette No. 131003/91, according to which transfer functions of a multi-input multi-output system for modeling characteristics of a chemical plant are estimated one after another to thereby reduce the degree of an identification model. With this known method, response signals are measured at a plurality of output ends upon each application of a test signal to one of input ends; no signals are applied to the other input ends at the same time. Since all of the transfer functions of the system can not be measured simultaneously, it is necessary to repeat measurements of response signals at the plurality of output ends for the input signal that is applied to each of the input ends.
As a solution to this problem, there is suggested in U.S. Pat. No. 5,661,813 a method for simultaneous estimation of all transfer functions by adding input signals with uncorrelated variations, or inputting N uncorrelated estimated or pseudo noise signals. The estimation of transfer characteristics of the N-input M-output linear system consumes much time because it is necessary to make sure, for all of Nxc3x97(Nxe2x88x921)/2 combinations of input signals, that the input signals are sufficiently uncorrelated. In addition, when a set of highly correlated input signals is found, it is necessary to uncorrelate the set of input signals by adding thereto different variations, in which case, however, the other sets of input signals need to be checked again for correlation.
When the N-input M-output linear system is driven by an identical signal or highly correlated signals, it is impossible with the prior art to guarantee identification of the transfer characteristics of the Nxc3x97M multiple transmission paths. Such a situation is encountered, for example, in a multi-input echo canceller of a multi-channel teleconferencing system. In the multi-channel teleconferencing system, speech of one talker picked up by a plurality of microphones at a remote place is transmitted as multi-channel signals from the sending side, and at the receiving side the signals are received and the speech is reproduced by multi-loudspeakers in an acoustic space where multi-microphones for sending use are placed. Because of a strong correlation between multi-channel signals generated by the same loud speaker, it is not usually guaranteed that the estimated transfer functions of the transmission paths between the multi-loudspeakers and the multi-microphones at the receiving side always coincide with the actual transfer functions even if residual echoes are cancelled.
It is therefore an object of the present invention to provide an apparatus and method which permit simultaneous estimation of transfer characteristics of multiple linear transmission paths irrespective of the correlation between simultaneous input signals thereto and hence avoids the necessity for checking their correlation, and a recording medium with the method recorded thereon.
According to the present invention, in simultaneous estimation of transfer characteristics of Nxc3x97M transmission paths of a linear FIR system defined by its N input points and M output points therebetween, N being an integer equal to or greater than 2 and M an integer equal to or greater than 1, N-channel input signals are processed by N pre-filters of different zero points to generate N-channel preprocessed signals, which are applied via N actuators to the N input points of the linear FIR system, respectively, then response signals from the linear FIR system are detected by M sensors at the M output points, and the transfer characteristics of Nxc3x97M transmission paths are estimated from the N-channel preprocessed signals and the response signals detected at the M output points.
Thus, the present invention allows simultaneous and separate estimation of transfer characteristics of Nxc3x97M multiple transmission paths from a variety of input signals.